text:
Do helicopters use more fuel when hovering?
This answer says

A helicopter uses a LOT more fuel hovering than it does in forward flight.

Is this correct? Why?

Very related: https://aviation.stackexchange.com/questions/13397/why-would-a-helicopter-climb-in-a-spiral

Obviously it uses a LOT more fuel per mile if it's just hovering, duh! :)

The simple answer to why it does is that the blades have less air to push on in hover. And moving less air faster takes more energy than moving more air slower.

Yes it is correct that helicopters use more fuel when hovering: the engine needs to apply more power to overcome drag. Here is a graph of the engine power required for different airspeeds, from J. Gordon Leishman, Principles Of Helicopter Aerodynamics:

The line for total power goes down between 0 - 70 kts with increasing airspeed, this is caused by the line for induced power: power required to overcome the induced drag of the helicopter blade. The total required engine power is the summation of:

Induced power. The power required to overcome the induced drag of lift creation, further detailed below. Propulsive power relates to the engine exhaust velocity now being helpful, and to the rise in induced power at higher speeds due to compressibility drag.
Profile power, required for the blade profile drag.
Parasitic power, for the drag caused by the air frame, rotor hub etc. Zero in the hover, very dominant at top speed. Helicopters have shapes that are way less aerodynamic than fixed wing planes, and this source of drag becomes very significant at higher speeds.
Tail rotor power. Up to 20% of the main rotor power both in the hover and at top speed, very low in the middle due to the helpful vertical tail. At top speed the torque of the main rotor is high and the tail rotor must do more work, unless the vertical tail can be adjusted.

Induced power is dominant in the hover. Induced drag is caused by the backwards tilt of the lift vector: the higher the angle between blade and free stream, the more the vector is tilted backwards, which causes both loss of lift and increase of drag. The equation for lift L is:
$$ L = C_L \cdot \frac{1}{2} \cdot \rho \cdot V^2 \cdot S$$
and at a given altitude, the two variables here are $C_L$ (lift coefficient) and $V$ (airspeed at the blade). $C_L$ is an approximately linear function of angle of attack at the blade, so lift increases linearly with blade tilt-back and quadratically with increasing airspeed over the blade.

Above graph from Leishman shows the velocity distribution over the blades when hovering, and at airspeed. Quite a complicated situation - when hovering, the airspeed reaching the blade is only the rotational speed of the rotor, at forward speed the blade going forward has rotational speed plus airspeed.
The helicopter does not roll over and both forward blade and retreating blade deliver the same amount of lift, with the rearward going blade tilted back more than it was in the hover. But the forward going blade is tilted back a lot less: airspeed has a quadratic influence.
Note that the circle in the plot at fwd airspeed is not stalled flow, but reverse flow: the air streams in at the back of the blade. So drag is now negative, the airstream helps to propel the blade! However there is loss of lift in the reverse flow area.
Induced power reduces with airspeed at first according to the simple 1-D impulse consideration (more air mass through the disk), and later increases as the disk is increasingly tilted forward and must do more work to overcome losses from rotor profile drag, airframe parasitic drag, and compressibility drag.
There is also an interference effect of the downwash over the fuselage: in the hover the air streams straight down, while in forward flight the rotor wash is more aligned with the fuselage, catching more of a streamline shape. Parasitic drag is of course dominant at top speed, while offloading the rotor by using fixed wing surfaces reduces the induced power at high speeds - but from hover to moderate forward speeds it is purely the reduction in lift induced power that creates translational lift.

Choppers are so technical !  Awesome.

This is only for going forwards though right? Does the same effect apply if the chopper is going sideways or backwards?

Yes it does, or if there is sidewind when it goes up and hovers in place. When there is sidewind the pilot needs to apply pedal to adjust anti-torque. With a tailwind the vertical stabiliser will make it hard to maintain heading.

Great detail here - thx for the speed/power curves.

Regarding induced power, and this statement: "the higher the angle between blade and free stream, the more the vector is tilted backwards, which causes both loss of lift and increase of drag ". Should it instead be: "the higher the angle between the rotor plane and the effective relative airflow, ..."? Reasoning: 1. The effective lift vector direction doesn't change with the blade angle (although magnitude likely does). 2. The resultant wind vector is what's ultimately used to calculate lift/drag directions. 3. The portion of the lift in the rotor plane is what's felt as "drag" on the rotor.

@Philip No it is the blades that determine the lift & drag characteristics, relative to the free stream airflow direction. The rotor plane is of interest in first instance, but is a resultant of the blade angle and free stream angle as well.

@Koyovis I agree that blade angle and free stream airflow direction are factors in the lift/drag "characteristics".  However, I'm specifically referencing your statement regarding induced power.  Can you explain how the lines of reasoning I gave above are incorrect, or perhaps how they don't back up the suggested changes?

Yes, it is correct, if the helicopter doesn’t fly too fast. A helicopter will produce the necessary lift most efficiently at a moderate forward speed.
In a hover all the airflow which is available for lift creation must be generated by the rotation of the main rotor. This means that a small amount of air must be accelerated by a lot. If the helicopter adds forward speed, it can achieve a higher mass flow through the rotor, and now less acceleration of air is needed to achieve the same lift. This improves the efficiency of lift creation. If the helicopter goes faster than its speed for maximum rate of climb, aerodynamic drag grows too high and reduces efficiency again.
At high speed, the tips of the advancing blades might reach transsonic speeds, which produces a noticeable drag increase, and the inner part of the retreating blade will see very little airspeed, and to still produce lift, the whole blade will pitch to a high angle of attack, causing the inner part to stall, which again produces a noticeable drag increase. There is a sweet spot between hover and fast speed where the required power reaches a minimum.

Yeah, I'm not a physics student, but I work on Black Hawks. If you conceptualise a helicopter as just a main rotor disc producing lift, then Peter Kampf's answer about mass-flow through the rotor disc is the greatest factor. (Remember the disc is tilted forward as the helicopter moves forward).
However, your question actually asked why do they burn less fuel: well, thousands of little design features on the airframe each help save precious pounds of fuel in forward flight. (You might want to do a Google image search to look at while you read this.)
The Black Hawk has a cambered vertical fin which unloads the tail rotor above 60kts, and this torque is redirected into the main rotor. It has a variable stabilator which changes angle with fwd airspeed (= changing main rotor downwash angle) in order to provide lift, further unloading the main rotor. The tail rotor is canted at an angle and spins backwards into the main rotor wash, again to unload the main rotor, freeing up more power for forward speed. It has on-flight computers and a mixer unit which flattens out the airframe in flight, so that it doesn't present a flat cabin roof into the airstream at high forward airspeeds. The flatter you can keep the disc into the relative airflow, the smaller the pitching angles of the blades, and the less parasitic drag from the rotor disc.
The main rotor blade tips are swept backwards to delay the onset of transonic tip drag as the advancing blade sees higher relative airspeeds in forward flight. Other helicopters have airframe fairings that generate lift off the cabin body in forward flight. All of these aerodynamic savings are present in forward flight, but not in the hover. And lastly, your turbine engine air inlets will benefit from some ram-air effect in forward flight, which means burning less fuel for the same torque. Every helicopter in the world uses some or all of these features to save fuel in flight, and if you compare generations of helicopters (Bell 47, Bell UH-1, Bell 412, Black Hawk), you can see these features gradually develop.
There are other considerations when a helicopter is hovering just off the ground, but I've tried to list just some of the ways helicopters are designed to save fuel in flight. Hope some of this helps.

talk about a real-world answer!  awesome

"There are other considerations when a helicopter is hovering just off the ground": I suppose it uses less fuel due to the surface effect (basically, an air cushion emerges which increases lift)? That would imply that if you need to hover for extended periods of time, do so close to the ground.

The concept is known as "translational lift".  When moving in forward flight, a helicopter's rotor disc acts a lot like an airplane's wing - it has a significant lift-to-drag ratio. The required thrust to maintain level flight is reduced by that ratio, and therefore necessary engine power and fuel flow are also reduced.  In hover, the engine+rotor system has to supply thrust fully equal to the weight of the helicopter.

Thanks very much for the simplest answer here that provides a basic understanding good enough for us laymen to quench our curiosity.

You're welcome @JoelMellon, thanks for the kind comment.

When in a hover, the air has more time to setup into an induced wash from further upwards that translates into higher down flow speed by the time the induced wash reaches the plane of the rotor. When in translational flight, the rotor is continuously moving into clean air, so the down flow speed by the time the air reaches the plane of the rotor is less than that of a hover. Power equals force times speed, in this case consider the power output to the air. In both cases, the force is the same (equal to the weight of the helicopter), but in a hover, the down wash speed through the plane of the rotor is greater than during translational flight, so the required power in a hover is greater than in translational flight, until the translational drag becomes an issue.
Another issue is tip vortices. In a hover, these can get quite large, again due to all the time for the vortices to set up and the rotor tips moving into the vortices induced by the other rotor tip(s). In translational flight, the vortices are "washed" off by the relative horizontal wind, reducing the size of the tip vortices.

Another point to consider is whether helicopter has supplemental wings. Rather famous example is Mi-24 family of attack helicopters, where weapon pylons works as wings.
"At high speed, the wings provide considerable lift (up to a quarter of total lift)."
At high altitudes with full load the recommended lift-off procedure is to gain horizontal speed so wings pick up some lift.

Damn, a 25% weight reduction is no joke.

If gravity were the only force acting on an aircraft, then at each moment in time the aircraft would be gaining certain amount of downward momentum. So to maintain altitude, the aircraft must transfer that momentum to some other mass (i.e. air). That is, there’s going to be some air that starts with zero velocity (in the simplest case) and ends up with some downward velocity. Since momentum is mass times velocity, the velocity that the air has to be accelerated to will be inversely proportional to the mass of air accelerated:
$velocity = momentum/mass$
However, the energy of that air is: $½mv²$
When we substitute velocity into that equation, we get:
$energy = mass \cdot (momentum²/2 \cdot mass²)$
One power of mass in denominator cancels out the mass in numerator, and we get:
$energy = momentum²/2 \cdot mass$
Thus, doubling the amount of air accelerated downward halves the energy required. When an airplane is traveling at high velocity, a large amount of air is coming into contact with its wings, meaning that it does not have to expend much energy to generate lift (of course, the faster it travels, the more drag it experiences, giving a lift-drag trade-off). A helicopter experiences something similar: when it is traveling horizontally, it naturally moves into new air. When it hovers, there’s less air to accelerate downward, and what air there is has to be pulled towards the rotor by the rotor’s own effort.

Comments are not for extended discussion; this conversation has been moved to chat.

@AdityaSharma Is it really a good idea to extensively re-write someone else’s answer?

@Koyovis No its not. I have only corrected the mathematical angle of the answer, while fully conveying the facts as intended by the original answerer.

@AdityaSharma I see a lot of phrases that the original poster did not write, yet their name is under it. Helping the author by changing the equations into Mathjax would be a good idea, correcting spelling mistakes etc. But extensive addition of phrases is not done on this site

@Koyovis I apologise for that. Long story short: Sophit asked me to fix the math, I did so - but quickly realised that the changes made to the answer were drastic. I still submitted my edit proposal and against all odds it got approved. Now, I still believe that the message of the answer is still presented as originally intended by the answerer, and that all of those added phrases are only there to support the new fixed math. If you believe that my edits have been too drastic, let me know if you recommend me to revert to an older edit with only the Mathjax addition.

I will then post a seperate answer of my own with the corrected math; good day!

@AdityaSharma That is what I would do for sure, yeah.

@Koyovis Will do it now  :-)

Let us continue this discussion in chat.


text:
How is directional control during takeoff and landing maintained in tailwheel airplanes equipped with skis?
Reading this question got me wondering: From what I've heard and read about taildraggers, brakes are needed during takeoff and/or landing rolls to maintain directional control and prevent a ground loop. At low airspeeds rudder authority is insufficient to keep the plane going in a straight line.
How is this achieved when skis are mounted instead of wheels? Is there anything special about skis that makes them more controllable  than wheels in these situations?

I have a fair amount of ski time on Cessna 180s and Citabrias. It's a bit like flying floats. The main thing is, you have no brakes.
For lateral control, the skis have varying amounts of lateral compliance depending on the snow compaction, so you slip and slide more or less for each situation, but generally, the laggy reactions makes it easier to control direction on takeoff and landing compared to wheels.  The compliance is like being on wheels on wet grass; instead of darting to the side when the plane swings, it just kind of sashays laterally giving you more time to make corrections.
You may have a ski on the tail, which provides a limited amount of steering control to the extent that the ski design is able to bite sideways (a little skeg on the bottom really helps).  A decent tail ski on lightly compacted snow provides pretty good steering control without having to use large blasts of power to blow the rudder.  Otherwise, you are dependent on large blasts of power to blow the rudder to turn.
On ice or very deep powdery snow, you will be depending more on power to turn, and this is quite a challenge because the shot of power to get the tail to swing to turn also accelerates you forward, and remember, you have no brakes.
Bottom line is you can't really maneuver in tight spaces when on skis without using huge bursts of power, and annoying the local owner whose airplane may be in your prop wash, so it requires a lot more preplanning and thinking things through.
Some airplanes do without a tail ski, keeping the normal tailwheel, and this makes it easier to maneuver on compacted snow where the tire has some bite.  On deep loose snow, the tailwheel becomes an anchor, and you have to lift it out of the snow with shots of power and elevator, on top of lots of rudder to try to maneuver.  That can get exciting.  A tailwheel-only ski plane in deep snow is a pain.  For aircraft with wheel-skis, you normally see a tail ski with a penetrating wheel to do double duty.
So, you have no brakes. So you have to do run-ups on the move, either in a big circle if you have the space, or on the takeoff "roll" if you don't.
Then there's landing at the end of a sunny warm day on a compacted runway and the temperature drops below freezing in the late afternoon as the sun sets, and the runway has a bit of a downhill slope to it, and it's essentially compacted snow with an ice glaze.  Did I mention that you have no brakes?  Been there, done that.

re : "(a little skeg on the bottom really helps)"-- was going to speculate in my answer that it would be only logical to have some sort of skeg to give the tailwheel ski a little extra "bite"-- great answer.

Sounds like reverse would sure have come in handy!

When that happened, it was a Citabria and I dimly remember (it was about 1978) using what little directional control I had to head for uncompacted snow on a flat area off to the side to get slowed down.  If the runway had berms on each side that prevented that sort of excursion, I would have ended up in the fence at the end.  Yeah reverse would be handy, as it would be on floats.  On floats you have to get good at shutting down with mixture while still 50 feet away and coasting up to the dock.

You maintain control using the rudder (and throttle)...
It takes a lot of power to taxi around on ski's, meaning you are creating a lot of wind with the prop wash. On wheels you can idle around and move, but on skis, even though you are on snow/ice, it still takes a lot more power to move you. If you want to change direction but go slow, you need to put in power, make the correction, then remove it.
Push rudder then quick blast of power.
It's reported that its difficult to ground loop an aircraft on skis (but not impossible). The aircraft tends to slide sideways rather than tip over.
Some aircraft also have wheels on the skis:

Source: GeneralAviationNews
And that may help by using traditional methods to maintain directional control, although if the wheels are in deep snow it probably does very little in that regard.

From what I've heard and read about taildraggers, brakes are needed
during takeoff and/or landing rolls to maintain directional control
and prevent a ground loop.

This isn't meant to be a complete answer to this question, but it might help inform your understanding of the differences between operating a tailwheel aircraft on wheels versus on skis to point out that there have have been many tailwheel aircraft lacking brakes entirely (e.g. World War 1 era aircraft.)  In many tailwheel aircraft, brakes really aren't an important part of maintaining directional control in most situations, except while taxiing.  Attempting to use the brakes to maintain directional control as a tailwheel aircraft is decelerating after landing can actually re-inforce any left or right "swing" of the nose that is taking place, especially in cases where the landing gear "track" (distance between the main wheels) is relatively narrow, and thus can tend to cause a ground loop rather than to prevent it.
Another point worth noting is that when a tailwheel aircraft is on skis, the tailwheel will often have its own small ski, which also helps with directional control.

(links to authoritative external sources discussing the best techniques to use in various tailwheel aircraft would be helpful here)

source: Stack Exchange

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sort: votes


text:
Does Mexico City's new airport follow international regulations?
The new Felipe Ángeles International Airport (NLU/MMSM) in Mexico started operations today, I think that's a good thing. But I was wondering, since the last time I saw info about it, the progress was at 68%. And there was lacking reviews and certifications (at least by ICAO), supposed there is no ILS operating or installed.
There is some turmoil about this station would conflicts with MMMX, and MITRE recommendation to not operate it.
But what surprised more its, indeed, there was already a few flights departing and arriving to MMSM, so isn't supposed that the carriers isn't allowed to operate in places with no certifications. Or in last instance, this isn't convenient.
I thought the secures refused to cover in situations without certifications of airworthiness?
Is this airport breaking the ICAO/FAA/EASA Directives?
If so, what are the sanctions?

Not sure what FAA or EASA have to do with this. They have no jurisdiction in Mexico.

Could you please cite the certifications this airport is lacking at the moment? Regarding the confilcts with surroundin airports, Wikipedia offers us this info: The Valley of Mexico will be the first in the country where the performance-based navigation system (PBN) is used, which will allow the Felipe Ángeles International Airport, Mexico City International Airport and the Toluca International Airport to operate simultaneously without the operations of one impeding those of the others.

There are plenty of non-ICAO airfields around the world, they aren't subject to sanctions, they just haven't signed-up to the ICAO standards.  A large airport operating outside the ICAO rules will probably not attract many international carriers but there's nothing to stop them trying.  https://en.wikipedia.org/wiki/International_airport

Is there any indication that the airport is not working towards getting the certifications? If so, it makes sense to me to open and start working out the kinks before "huge" amounts of traffic start arriving (presuming that's the goal). Just look at all the issues Denver had with their automated baggage handling system - good thing they tested that before it started loosing/destroying paying passenger's bags...

Related: https://aviation.stackexchange.com/q/91164/35166

The only significant “international” regulation is the Chicago Convention, which is deliberately broad and vague because that’s the only way everyone would agree to it.
ICAO creates recommendations to national CAAs, and then the CAAs modify them as they see fit to create their own regulations. The Chicago Convention does address whose rules apply when an aircraft or pilot from one member state operates in the airspace of another member state, but in practice the rules are close enough (and violations so infrequent) that it rarely matters.
So, ICAO is not “certifying” anything in Mexico, because they don’t do that, nor do the FAA or EASA, because they don’t have the jurisdiction to do that. It’s up to Mexico’s CAA to certify airports within Mexico.
That said, if foreign operators (or their insurers) don’t trust Mexico’s CAA, then they will choose not to operate there, and they might have discussions or agreements about such choices via industry (not regulatory) bodies such as the IATA. So, if Mexico wants foreign operators to use their new airport, their CAA will take into consideration any concerns those operators raise, which is why most CAAs adhere fairly closely to ICAO (and IATA) recommendations in the first place.

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What's the difference between an accelerated stall and a dynamic stall?
I am a commercial pilot. I always like to understand everything in detail, so here's my question:
What's the difference between an accelerated stall and a dynamic stall? For example, in a high-speed low pass, if we yank the yoke back, which of those two stalls may occur? It seems to me that what happens'll be a little bit of both. Also, can a dynamic stall occur at less than the normal stall AoA if the pitch rate is very fast? And, last but not least, is an airplane taxiing on the ground considered stalled (the stall AoA is not reached, but the lift produced is insufficient to fly)?

Welcome to aviation.SE! I changed your question title to make it a little more specific because we prefer to have only one question at a time (the [tour] may be helpful to see how the site works). For your last question about what "stalled" really means, this question might be helpful.

Ok but how can the critical AoA be exceeded other than pulling Gs? To me it looks like the same thing. The only difference is that if the rate of roll is really fast we can get a brief moment of augmented lift before falling into deep stall.

related: https://aviation.stackexchange.com/q/61750/1467, also mentioned in https://aviation.stackexchange.com/q/6366/1467

The two stall conditions are both under different circumstances than the low speed, static stall conditions from exceeding critical Angle of Attack (AoA), they are not one and the same however.
Dynamic stall is described in this article from the Ohia State University, in relation mainly to helicopter rotors:

Aerodynamic bodies subjected to pitching motions or oscillations exhibit a stalling behavior different from that observed when the flow over a wing at a fixed angle of attack separates.

Further reading in this article from the University of Adelaide. The stall behaviour is described as Dynamic Stall, and tied to a pitch rate of the profiles.
Accelerated stall is mainly referred to for aeroplanes subject to g-forces, such as described in this answer on this site.
So when yanking the yoke back during a high speed low pass, creating higher g-forces, Accelerated Stall seems to be the more appropriate term. The situation is of course dynamic as well...
The Ohio State article mentions dynamic stall occurring in a windmill, which is of course not an accelerated stall. So not all dynamic stalls are accelerated stalls.

Also, can a dynamic stall occur at less than the normal stall AoA if the pitch rate is very fast?

The Adelaide University article describes a pitch-up rate temporarily increasing $C_L$, then developing deep stall. The pictures show a leading edge vortex developing at 40° AoA (figures 3d and 3j), way higher than static stall angle. Pitch-up movement seems to delay the stall, until it occurs very dramatically without any warning.

And, last but not least, is an airplane taxiing on the ground considered stalled (the stall AoA is not reached, but the lift produced is insufficient to fly)?

No it is not considered stalled. Stall is a condition of separation of the boundary layer, which does not occur during taxiing.

The definitions are not exclusive, so don't have to chose the more appropriate term. You simply use both if both apply.

@JanHudec hence the sentence: "The situation is of course dynamic as well..."

+1 for pointing out that the stall AoA in a dynamic stall is higher than in a slow, quasi-static stall.

There are several questions here, I will answer two of them.
A dynamic stall occurs when the wing's critical angle of attack is suddenly exceeded, as in the case of a sudden yoke pull & pitch-up.
An accelerated stall occurs when the wing loading is suddenly increased by g-forces, as in the case of adding back pressure in a steep turn.

Your distinction between the two seems to be based on wings level vs banked, and rate of onset.  Would you agree with that observation?

Well, if the plane is in a coordinated turn, the wings are banked and as a result the wings are heavily loaded by the g-force that results. This seems like a chicken-and-egg situation and I do not know how to resolve it, but I'll bet peter kaempf does... -NN

I'm not sure what there is to resolve, it's just a terminology question.  I would actually say they are the same thing.

I agree.  They are both exceedances of stalling AOA induced by acceleration above 1G.


text:
What exactly are these frequency uses mentioned in the FAA's advisory circular on VHF use?
The FAA's document AC 90-50, (currently AC 90-50D) sets forth how the aviation communications frequencies (118.000-136.975) are allocated in the United States.  Many of the assignment designations are plain enough, and common to the understanding of pilots, but some designations are outside normal use, for example:

Aviation Support
Domestic VHF
Flight Test Stations
Flight Inspection Use

Is there a document which provides descriptions of what these designations are and what qualifies as legitimate use of those frequencies?

The FCC is the ultimate authority on radio frequency usage in the US. 47 CFR § 87.5 has the definitions of the aviation services.

Aviation support station. An aeronautical station used to coordinate aviation services with aircraft and to communicate with
  aircraft engaged in unique or specialized activities. (See subpart K)

Subpark K:

§ 87.319 Scope of service.
Aviation support stations are used for the following types of
  operations:
(a) Pilot training;
  (b) Coordination of soaring activities between gliders, tow aircraft
  and land stations;
  (c) Coordination of activities between free balloons or
  lighter-than-air aircraft and ground stations;
  (d) Coordination between aircraft and aviation service organizations
  located on an airport concerning the safe and efficient
  portal-to-portal transit of the aircraft, such as the types of fuel
  and ground services available; and
  (e) Promotion of safety of life and property.

Domestic VHF use is described in § 87.263. In general, these frequencies are used for communicating with aircraft, or between ground stations. There are various restrictions on certain frequencies/ranges, and despite the name, some are allowed for use on international routes.
Back to § 87.5:

Flight test aircraft station. An aircraft station used in the testing
  of aircraft or their major components.
Flight test land station. An aeronautical station used in the testing
  of aircraft or their major components.

Flight Inspection is not defined in § 87.5, so it may be the FAA setting these aside for their own use, but these frequencies are used by the FAA for ensuring "the integrity of instrument approaches and airway procedures."


text:
What is the history of the CFI spin endorsement requirement?
FAR 61.183(i)(1) requires that a Flight Instructor candidate receive a logbook endorsement certifying proficiency in slow flight, stalls and spins.
How long has the requirement for spins been in the regulations?
Is it possible that there are CFIs in the field that do not have this endorsement?

It used to be a requirement that all PPL candidates practice spin training, but the FAA decided that more people were being killed doing spin training than spins in practice, so they removed the requirement from the PPL but left it in for instructors (because students do strange things during practice that can cause spins). I'm not sure what the regulatory history is though, if that is what you are asking.

I can't find a definitive answer but it looks likely - but not certain - that endorsements were introduced sometime between 1980 and 1991.
An AOPA article says that spins became an instructor-only thing in 1949, per CAR Amendment 20-3 (which I can't find online). That doesn't say anything about when an endorsement was introduced, though.
AC 61-67C - Stall and Spin Awareness Training has some historical information on page ii, including this (emphasis mine):

In January 1980, the Federal Aviation Administration (FAA) announced
its policy of incorporating the use of certain distractions during the
performance of flight test maneuvers. This policy came about as a
result of Report No. FAA-RD-77-26, General Aviation Pilot Stall
Awareness Study, which revealed that stall/spin related accidents
accounted for approximately one-quarter of all fatal general aviation
accidents. National Transportation Safety Board (NTSB) statistics
indicate that most stall/spin accidents result when a pilot is
distracted momentarily from the primary task of flying the aircraft.
Changes to part 61, completed in 1991, included increased stall and
spin awareness training for recreational, private, and commercial
Pilot Certificate applicants. The training is intended to emphasize
recognition of situations that could lead to an inadvertent stall
and/or spin by using realistic distractions such as those suggested in
Report No. FAA-RD-77-26 and incorporated into the performance of
flight test maneuvers. Although the training is intended to emphasize
stall and spin awareness and recovery techniques for all pilots, only
flight instructor-airplane and flight instructor-glider candidates are
required to demonstrate instructional proficiency in spin entry,
spins, and spin recovery techniques as a requirement for
certification. Part 61 was extensively updated in 1997.

In version A of the Flight Instructor PTS (May 1991), candidates are required to demonstrate a spin but the examiner can accept an endorsement instead, at their discretion (pp. 2-xliii - 2-xliv):

At the discretion of the examiner, a logbook record attesting
applicant instructional competency in spin entries, spins, and spin
recoveries may be accepted in lieu of this TASK.

The task itself is described as:

Demonstrates and simultaneously explains a spin (one turn) from an
instructional standpoint.

So we can say for sure that since 1991 at least, instructor candidates have been required to either spin the aircraft during their checkride or have an endorsement saying they were capable of doing it. (I say "since 1991" because the same wording is still in the CFI PTS today.)
Of course, getting the endorsement requires spinning anyway (see AC 61-67 again), meaning that the candidate always had to spin an aircraft in order to qualify. The endorsement wasn't a way to avoid doing that.
I can't find any earlier sources or PTS versions that show the pre-1991 requirements, maybe someone else will have more luck.
And to answer your second question, 1991 was 'only' 29 years ago. It seems very possible that there are instructors who have been active for longer than that. If you're wondering if some instructors out there may never have spun an aircraft, the AOPA article implies that they've 'always' been required to spin aircraft anyway, regardless of exactly when the endorsement became an alternative to doing it in the flight test.


text:
Why do we have to check the speed brakes during GPWS memory items on an A320?
Just out of curiosity, I want to ask why do we have to check that the speed brakes lever is retracted during a GPWS (Ground Proximity Warning System) maneuver using the A320 memory items?
It clearly says in the F/CTRL SYS that the SPD BRK is inhibited if the thrust levers are above the MCT detent.

Because that is how American Airlines Flight 965 went down.
Long story short, the pilots deployed the speed brakes to increase descent rate during approach, and did not notice they were off course. The GPWS warning sounded. The pilots reacted within one second. They did everything they could - except they did not retract the speed brakes. They crashed.
Why is it a memory item? Because GPWS means imminent terrain contact. You don't want to wait around for some automatic mechanism to retract the speed brakes for you. Let alone take the chance of that mechanism failed. You do it. NOW.

But that was a Boeing 757, not an Airbus. The A320 should automatically retract the speedbrakes.

@Bianfable yup, it should. But when the GPWS red warning light is flashing, it is not time to take chances. And it doesn't really take that long to execute, so there is no downside. The glance takes a split second. And if needed, the handle is right next to the throttle.

@Bianfable your comment on Dave's answer mentioned the speedbrakes stay retracted until the inhibition condition disappears, I'm guessing the logic in checking the lever is to make sure that speedbrakes don't pop back up again unintentionally once inhibiting conditions are no longer present.

@ymb1 that's not what I'm after. I was referring to Bianfables comment on Dave's answer, as it gave the impression that speedbrakes would automatically extend back if they were retracted by GPWS, and the "situation" was normalized and speedbrake lever was still in extended position.

Ok I think I understand now @ymb1:  the GPWS retracts and disables the speedbrakes, and activating the speedbrake system requires the reset you described.

I think you are missing a key term, I don't have the official SOP's on hand but many of the secondary references are using the same verbiage:

Speed brake extension is inhibited under the following conditions...

Either thrust lever is above MCT

In other words you cant extend the speed breaks if the throttle is over the MCT but if they are extended they may remain that way.
So why is it a memory item... If you are to low or in danger of hitting an obstacle the general solution is to CLIMB and the best climb rates wont be achieved with speed brakes out as they generally spoil lift so in the event of a GPWS alarm you will want to retract the speed breaks and climb.

The FCOM says "If an inhibition occurs when the speedbrakes are extended, they retract automatically and stay retracted until the inhibition condition disappears [...]." And "Thrust Levers above MCT position" is listed as an inhibition condition, so they should retract automatically.

@Bianfable Is there a case where you would climb with the throttles not in MCT?

I found the steps in this QRH online (see p. 66) and they clearly say "THRUST LEVERS ... TOGA, SPEED BRAKES LEVER ... CHECK RETRACTED", so they are above MCT and one should still check the speed brakes for some reason.


text:
When can I descend on a feeder route if instructed “maintain ... until established”
AIM 5-4-6 discusses how Approach Clearance applies to feeder routes. It says that when ATC clears an aircraft for the approach, the aircraft is expected to use a feeder route if applicable. My understanding is that this includes the altitude published on the feeder route.
However, if ATC says “maintain 2000 until established, cleared approach” and the aircraft is approaching a feeder route, when would the aircraft descend to a published altitude? On the feeder? Once past the initial approach fix?

Is this something that you have experienced yourself? If so an example would be helpful. Approaching a feeder route and hearing "Maintain... until established" imply two separate situations, to my ear.
From a controller's standpoint, if you are operating on an unpublished route (radar vectors or direct the IAF or similar), we follow the 7110.65 4–8–1b:

b. For aircraft operating on unpublished routes, issue the approach clearance only after the aircraft is:
1. Established on a segment of a published route or instrument approach procedure, or
2. Assigned an altitude to maintain until the aircraft is established on a segment of a published route or instrument approach procedure.

So if you are flying toward the airport but not on any airway or feeder route, you will receive vectors to final, or to an initial fix. Then in accordance with b2 we can say "Maintain 2000 until established on final approach course, cleared VOR runway 18 approach." You descend and maintain 2000, and do not descend any more until you (as PIC) deem yourself established on the published approach segment. (The procedure is slightly different if you are with a Terminal Arrival Area designated for an RNAV approach and above the minimum altitude for that TAA; then we can just say "Cleared RNAV runway 18 approach.")
If, however, you are operating on a published route, such as an airway leading to a feeder route, all we have to say is "Cleared VOR runway 18 approach." You are currently at a safe altitude on the airway, and the approach is specifically designed to give you all the information you need to get from that airway altitude down to 0AGL without running into any terrain. If this involves the use of a feeder route, you are expected to use the feeder route and any altitude restrictions for it.

Essentially: I don't think you would be told "Maintain 2000 until established, cleared approach." Perhaps you would be told "Maintain 2000 until FIXXX, cleared approach" where FIXXX is the beginning of a feeder route and you will intercept FIXXX at an angle of 90º or less; you are then assumed to be established on a published approach segment once you cross FIXXX. Once you are established (i.e. after FIXXX) you may descend in accordance with the published approach procedure.

I have not experienced this, but an instructor brought up this possibility during a discussion on feeder routes. He was either trying to confuse me or simply confused himself. Thanks for clarifying!

@asb1230 this answer may also help.

@asb1230 I just realized I linked your own answer... well... I still think that answer provides some helpful information about this situation.


text:
Do helicopters have automatic tail rotor compensation for anti-spin?
Helicopter pilots need to push on the pedals constantly to compensate for the main rotor torque, so that the helicopter does not start an unintended spin. But it would be annoying to indefinetly put pressure on the pedal. Do helicopters  have an autopilot that automatically keeps them stable ?

Are you talking about in steady forward flight or slow flight/hover? Why do you believe that you would need to apply pedal to stabilize the helicopter?

Related: Do rotorcraft like helicopters have autopilots?

Also: https://aviation.stackexchange.com/questions/23977/why-not-simplified-fly-by-wire-helicopter-controls

I read this more like a question on flight control trim forces than on an autopilot.

Autopilots control the flight path, they usually concern themselves with the travel of the centre of gravity of the aircraft. Stabilising the aircraft is more the job of the human pilot. In a helicopter that is indeed not an easy job - larger helicopters have a mechanical mixer that helps the human do that, a set of lever arms that input extra tail rotor as collective is applied, for some automatic compensation of the increased torque of the main rotor.
Having to indefinitely put pressure on a flight control is an annoying issue for fixed wing pilots as well, that is why they can be trimmed. Stick and pedal forces can be trimmed away, so that the neutral point of the control becomes the point of zero force: once the control is trimmed, you can take your hands or feet off of it and it stays there.
In fixed wing aircraft, trim is usually adjusted with a flick switch on the handle. Flick it sideways and the roll control spring neutral point moves, flick it forwards and the pitch spring neutral point adjusts. Helicopters have such a switch as well, and that works well for fine control in cruise. But in the hover there is a lot of swishing action of all flying controls, and the beep trim switch is just too slow to follow this. Spring forces that are normally helpful, are now a hindrance.
That is why helicopters in addition have a trim on/off button: when selected, all feel forces cancel and the stick or pedal is very light, only a bit of friction and damping. What happens is that the feel forces neutral point automatically follows the stick position. Once the helicopter is stable, the trim forces can be selected ON again, and hands and feel can be removed - although I believe this is never a good idea in a helicopter.

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text:
How much lift is lost by wing dihedral, and can it be fixed?
The loss of lift, though negligible, was mentioned in another thread regarding the angle of dihedral wings. Not knowing the coreect name but will external "ribs" on the wing surface reduce that loss? Also, is that lift loss less or greater with forward swept wings than straight or rear swept? And is that loss due to the dihedral angle of the wings increase with a greater angle?

The title of your post should reflect your actual question.

The amount of lift lost to dihedral is because of a trigonometrical condition, and cannot be fixed by ribs.
Lift is always perpendicular to the local surface, and each wing half develops lift at the dihedral angle. Only the vertical bit is useful for suspending the aeroplane weight.
In order to quantify:

Cos 5° = 0.9962, so less than 0.4% of lift is not useful.
Cos 10° = 0.9848, meaning a 1.5 % loss.

Wing sweep has the same roll/sideslip stabilising effect as dihedral does (this answer), so less or no dihedral is required if the wing is swept. But there are many other considerations related to lift and drag with a swept wing.

Dihedral is included in many wing designs because it is very useful in making the plane fly in a stable, comfortable manner.  As Koyovis explained, lift loss is minimal, and if more lift is needed, simply make the wings larger (or reduce weight!).  This is called wing loading.
Is far as external "ribs", these are called wing fences, and are not usually needed on top of the wing unless you plan to fly near the speed of sound.
However, wing fences underneath the wings, at the wing tips, are very useful for slow flight.
These are seen with many Short Take Off and Landing kits.  Bottom wing tip fences, along with flaps and slats, help increase the Lift Coefficient and maximum Angle Of Attack of your wing.


text:
How does Mach number of Jet engine vary with respect to Compression Ratio?
How does the performance of jet engine vary with Compression ratio.
What are the variables that affect the Mach number of Jet engine.

The performance and exhaust Mach numbers depend on very many variables, to name a few:

Inlet shape.
Compression ratio.
Combustion inlet velocity of the airflow.
Throttle setting
Turbine entry temperature.
Exhaust shape.

A multi-dimensional matrix which to fully describe takes a book like this one. My edition has 491 pages, and the scope of the answer would be very broad indeed.
The exhaust Mach number however, is mainly determined by the shape of the exhaust. If the exhaust is convergent only, maximum Mach number of the exhaust stream is M = 1. In order to reach supersonic exhaust velocities, the shape must be converging then diverging. If converging only then depending on the operating state the exhaust can be choked, at a higher pressure than the environment. The higher pressure will still contribute to thrust.

Figure above shows an example for a supersonic exhaust for a low bypass engine.


text:
Why is it, with less airflow, you'll have less control effectiveness?
I understand the principle of less airflow, less control, but why is that the case?

A control surface deflects air. So the more air deflected, the more...?

How is this not intuitive?  With zero airflow, controls just sit there & flap.  With a tiny amount of airflow, the control might accomplish a little bit.  With lots of airflow, the control has lots of moving molecules to work with.

@RalphJ In practice, "intuitive" is hard to identify or even define (YMMV). I'd say this is a naive question, but not a bad or unclear one. Although, depending on how much detail the OP needs, it might be better on physics. SE.

When deflected, the control surfaces (ailerons, elevator, rudder) cause an aerodynamic moment about the Aerodynamic Centre. A moment has a moment arm and needs to have a length reference - the aerodynamic moments are defined with reference to wing dimensions: wing span for rolling and yawing moments, and Mean Aerodynamic Chord for pitching moments. If we have a look at the pitching moment P:
$$ P = C_{r_{\delta e}} \cdot \delta_e \cdot q \cdot S \cdot MAC$$
With:

$C_{r_{\delta e}}$ = elevator coefficient (dimensionless)
$\delta_e$ = elevator deflection
$q$ = dynamic pressure = $\frac {1}{2} \cdot \rho \cdot V^2$
$A$ = wing area
MAC = Mean Aerodynamic Chord

$C_{r_{\delta e}}$, A and MAC are constants. So: pitching moment of the aircraft is proportional to elevator deflection, and to the square of the airspeed. Fly twice as fast, and the pitching moment from a certain elevator deflection will be four times as high.

Because moments of inertia don't change with speed
Control effectiveness means that the controls effect a change in the balance of moments which results in the desired attitude change. The smaller the control deflection for the same change in attitude, the higher their effectiveness. If $\ddot{\Theta}$ is the pitch acceleration, $∆F_H$ the force change on the horizontal tail due to a control deflection, $x$ the lever arm of that control around the center of gravity and $I_y$ the moment of inertia around the lateral axis, the formula for $\ddot{\Theta}$ is:
$$\ddot{\Theta} = \frac{∆F_H\cdot x}{I_y}$$
Both $x$ and $I_y$ are fixed, so only $∆F_H$ has the potential to increase pitch acceleration. $∆F_H$ is proportional to

Deflection angle $\eta_H$
Tail size $S_H$ (again fixed)
dynamic pressure $q = \frac{v^2\cdot \rho}{2}$

A given object will change its attitude more quickly when more force can be created. Therefore, more speed $v$ means more force change and a higher angular acceleration for the same deflection.

Basically what keeps your plane suspended above the ground despite gravity pulling it to the surface is the fact that your aircraft constantly pushes (and pulls) air molecules downwards; one of Newton's Laws says that this generates an equal and opposing (i.e. upward) force on your aircraft.
In straight and level flight this force is due to the positive angle of attack that the wings make with the relative wind (NOT THE FLIGHT PATH) which essentially forces air molecules downwards: molecules below the wing are deflected dowards along the bottom of the wing while molecules above the wing are pulled downwards along the top surface of the wing as it moves through them. When you go slower you deflect fewer air molecules downwards per unit time which demands a higher angle of attack in order to keep you suspended; this generally translates to more elevator deflection needed on the pilots part, or in other words: your controls are less effective.

This explanation of lift is incorrect - lift is generated due to Bernoulli's principle and the creation of a low pressure region on the top of the wing due to wing camber.

Certainly too: high pressure below wing deflects air molecules below downwards, low pressure above the wing "sucks" molecules above downwards. Bernoulli's principle is oft misunderstood by laymen, requiring careful consideration of the Lagrangian (rather than Eulerian) interpretation of streamlines.
See also
Https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

@ANDY-S No. This is a fine intuitive explanation, alluding to conservation of momentum. Bernoulli is just a different way of looking at things, and the results are equally valid. Not so your remark on camber - a symmetric airfoil can easily generate lift, and regardless, lift due to camber can also be explained with Bernoulli.

@ANDY-S you may read this section to understand that many principles are to take into account to explain lift generation

I fully agree that a wing produces lift using several different methods - including upwash, downwash, conservation of momentum etc.  I was attempting to clarify the common misconception that downwash lift is the primary lift generator on a wing.  Fully aware that an explanation of what causes lift would be an entire website in and of itself.  Also, @Sanchises, yes, camber is critical to lift generation.  A symmetric wing produces 0 lift at 0 AOA.  It is the introduction of AOA that creates a pseudo-dissimilar shape of the wing

This can be explained by Newton's second law, $F = m\times a$ and third law, every force has an equal force to the opposite direction.
$m$ here is the mass of airflow, $a$ is the acceleration caused to airflow (seen as changed direction of the airflow). A force equal to $a\times m$ is exerted to the control surface. More airflow, more mass, more force.
The very same reason why an airplane stays in the air in the first place.

You should enhance your answer by expanding some sentences. One important feature is the dynamic pressure. You may include it in your answer. You may also explicit your last sentence.

Control surfaces are used to change the effective camber of the airfoil they are controlling.  For example, a downward deflected aileron would increase the effective camber of a wing along the aileron's span. An increase in camber will increase the lift generated at a certain airspeed over that area of the wing, causing the desired rolling moment.  It is PARTIALLY due to this change in developed lift that generates adverse yaw, requiring rudder to coordinate turns.
At higher airspeeds, the wing is producing more total lift, and therefore more responsive to changes in camber.
Additionally, control surfaces also respond according to Newton's 3rd law - the ailerons deflect the passing airflow in a direction other than parallel to the wing skin, resulting in a reactive force causing roll.  As with the camber change, this phenomenon becomes more pronounced at increased airspeeds, and conversely less pronounced with a reduction in airflow.
A simplified explanation can be found at FAA Pilot's Handbook

I think OP wants to know the WHY behind the theoretical equations rather than simply THAT "Hey look when V increases in this equation it must be true that L also increases, c.p."

Control authority comes from the size of moments you can generate, which result from forces acting on the plane (the elevator, the ailerons or rudder), which come from pressure differences, which have a squared relation to velocity. If the airflow speed halves, your control authority gets cut in 4. If the airflow speed doubles, you get 4 times the control authority, etc.
Here's further explanation if anything isn't quite clear.
For control authority, you need to be able to apply your desired moment to the aircraft. Moments are forces acting at some distance from your rotation center. In an aircraft, say you want to roll the aircraft. The ailerons deflecting create a pressure difference between the right and left wings. This ends up as different forces acting basically at the ailerons, creating that roll moment. That's just the basics of roll. Now, for the airflow part.
First, I mentioned that for roll, it's those pressure differences caused by airflow over the wing and the aileron. The forces (the ones we're concerned with here) are created by the pressures on a surface. Remember, pressures are forces over areas. Now, lets look at the pressures. The equation for dynamic pressure is $\frac{\rho V^2}{2}$, that's the density times velocity squared over 2. We will assume our density isn't changing here, so in order to change the pressure, we change the velocity of the flow. BUT, its squared. Without airflow, it's obvious that no roll moment is created because the velocity is zero. A plane on the ground with no airflow over the wing doesn't try to roll.
In general, for roll, pitch and yaw authority(that's all of them), you can consider the feeling when you put your hand out of the window in a moving car. If you deflect air downward, your hand gets pushed up. In reality, it's the difference in pressure between the top and bottom, due to flow speeds. The faster you go, the more airflow, the greater the pressure differences you can generate, because of the squared relation. The slower you go, any flow speed differences might become negligible, meaning no pressure difference, thus no force acting.
With some numbers, let's say that at a high speed, the elevator gets deflected. Let's say that the flow over the top is going 100 (arbitrary velocity units), and the flow under is going 110. The pressure on top will be $\frac{\rho}{2}*100^2 = \frac{\rho}{2}*10000$ lets ignore the $\frac{\rho}{2}$ term, and just be aware that it linearly converts our number into a pressure. so we have 10000 pressure somethings on top, and we have 12100 pressure somethings on bottom (using the same formula). That means we have a net of 2100 pressure somethings pushing up on the tail now. Great, the tail has enough control authority to push the nose down as commanded.
Now, lets slow the speeds down by a factor of ten. The top air is going 10, and the bottom now goes 11. Lets see the pressure change compared to before. The pressure on top will be 100 pressure somethings, and on bottom it will be 121. The resulting net pressure acting on the tail is then 21 pressure units, 100 times less than before, even though the speeds only changed by a factor of ten. now, you have 100 times less force acting on the tail (resulting in equivalently less moment), and you might not be able to control the pitch as much as you want to.

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text:
Are there any duty time limitations on ferry pilots?
What FAA regulations are in force when a commercial pilot is being compensated to ferry a newly purchased aircraft?  Is there a limit on duty time?

Assuming that the ferry flight is a single-pilot operation under part 91 then this question answers it. If that doesn't help, please edit your question to add the regulations that will apply to the flight (91, 135, 121 etc.), the number of pilots and the type of aircraft.

@Pondlife I would think ferry flights would be flown with airline crew under Part 91.  Mentourpilot has a video about his new 737 ferry from Seattle to England on this.

@MikeBrass Ferry flights aren't just for airlines. Private aircraft are often ferried to the buyer as well, especially a buyer who needs instruction in an unfamiliar type and therefore shouldn't fly it home themselves.


text:
What are the minimum requirements for a student to solo a second make and model aircraft?
Suppose I've done my Private Pilot training in aircraft Make/Model "A" and the one I have access to breaks two weeks before my checkride.  My instructor also teaches in model "B".  What do I need to do solo in model B, if all of my endorsements are for model "A"?

Is it necessary to complete a Pre-solo exam?
Is it necessary to demonstrate all the maneuvers in 61.87?
Is it necessary to demonstate all the maneuvers in 61.93 for solo cross country privileges?

As long as your training has been done in the same category, class, and type as the aircraft you are switching into, the FARs aren't specific regarding repeat training required.  Instead, this will fall into the realm of flight school policy, and CFI judgement.
For example, if you are transitioning from a high wing to a low wing, or from manual to electric flaps, your instructor will need to ensure that you are safe and comfortable flying the new aircraft before signing you off to solo.  To the first question, it is almost a 100% certainty that you will at a minimum need to complete a presolo exam on the new aircraft to ensure that you know the systems, airspeeds, etc.
If the instrumentation and navigation equipment are similar enough you probably won't need to refly any cross country flights, but again this is at CFI/Owner's discretion.
You will, of course, need a new solo endorsement for the specific make and model.
And don't forget the most important piece, two weeks before your checkride YOU need to be comfortable in the aircraft you will take your practical exam in!  Don't short change yourself, push out the checkride if needed to make sure you are ready.
The best advice is to consult your CFI and/or the flight school chief pilot to put together a re-training/recovery plan to transition.  Good luck!
ADDENDUM:  I see in your profile that you are a CFI with 20 years of flying experience.  What gives?!  (downvote for what I perceive as deception...)

It's not quite that simple: assuming that the OP holds a student certificate then 61.87 requires knowledge and training in a "similar make and model" before a student can solo an aircraft, plus a logbook endorsement for the specific make and model. You're right that a lot is left to the instructor's judgment, but there are still requirements in the regulations.

Nothing I said contradicts 61.87, and I assume that the OP and their CFI have referenced this section.  (or will...)  My answer simply provides a perspective that supplements the CFR.  I don't feel the need for hand holding by posting quotes from this section, but can if it will strengthen the answer.  Link to the CFR if it helps:  https://www.law.cornell.edu/cfr/text/14/61.87  BTW, I added a specific line about needing a new endorsement for make and model to clarify this hard requirement.  "Similar" is a bit more squishy...

Short version: you don't need to repeat everything but you do need some kind of written test plus training and endorsements for the new aircraft. Your instructor has a lot of freedom to decide exactly what you have to repeat or not.

Assuming that you're a student pilot training for ASEL (airplane single-engine land) then 14 CFR 61.87 applies to all solo flights:

A student pilot may not operate an aircraft in solo flight unless that
  student has met the requirements of this section. The term “solo
  flight” as used in this subpart means that flight time during which a
  student pilot is the sole occupant of the aircraft [...]

The whole thing is too long to quote here and you can read it yourself; the general idea is to make sure that you're competent in the "make and model" (not the category and class) of aircraft that you're going to solo.
Here are some key points:

87(b) requires a knowledge test that includes operational limitations of the aircraft
87(c) requires logged flight training, and maneuvers
87(n) requires an endorsement for the specific make and model
87(p) requires a 90-day endorsement from an instructor

The regulations do give instructors some discretion: some of them refer to a "similar" or "appropriate" make and model, i.e. not everything you've learned has to be repeated in the new aircraft.
With all that in mind:

Is it necessary to complete a Pre-solo exam?

Yes. 87(b) requires a knowledge test including "Flight characteristics and operational limitations for the make and model of aircraft to be flown". Your instructor can decide exactly what that test covers (or not).

Is it necessary to demonstrate all the maneuvers in 61.87?

No. 87(c) requires training in an "appropriate" and "similar" make and model, but it doesn't require it in the specific make and model that you're going to fly. Your instructor determines what's similar enough or not.

Is it necessary to demonstrate all the maneuvers in 61.93 for solo cross country privileges?

No. 93(e) just requires that you log training, it doesn't say anything about the make or model. But note that 93(c) and (d) do require a current endorsement for the specific make and model.


text:
What are the advantages of the Riblett airfoil over the Original Pietenpol Airfoil?
Some builders have opted to build a Riblett airfoil based wing for the Aircamper rather than using the original Pietenpol wing.  What are the basic characteristics of the Riblett wing which differentiate it from the original wing, and what are the pros and cons in construction and performance?

Assuming that the same wing chord is used for both, one technical difference between the Pietenpol and the Riblett is the depth of the airfoil.  The Riblett that has been used on the Air Camper is the 612 (or sometimes the 613.5), which makes it 12% (or 13.5%) deep.  With a 60" chord, that's a depth of 7 to 8".  The deepest section of the stock Pietenpol airfoil is only 6-9/32", which makes it roughly a 10.5%.  The offshoot is that a taller spar can be used with the Riblett, and in spar strength the height makes a great deal of difference because the section modulus goes as the square of the height and the section modulus is what determines bending strength.  A taller spar can be made lighter with proper design.  Another useful consequence of using the Riblett is that a deeper wing section allows a larger fuel tank to fit in the space between the spars.  The stock Pietenpol wing centersection can accommodate maybe 10 gallons, but the Riblett should be able to fit more up there.
I've attached a graphic overlay comparison of the Pietenpol airfoil with the Riblett GA30U-612; Piet in red, Riblett in white.  The 613.5 is similar but the differences are a bit more accentuated.
For performance differences, I will defer to both a technical treatise by Mike Shuck (on the airfoil list) and the flight comparison between two very similar Piets conducted several years ago and documented in the BPA Newsletter.  Very similar performance in all flight regimes.


text:
Why doesn't Ketchikan, AK have a control tower?
Why doesn't Ketchikan International Airport (PAKT) in Alaska have a control tower?  The special flight rules applicable to the area (14 CFR 93 Subpart M) seem to make it clear that traffic management is an issue, focusing mainly on being in contact with the Ketchikan flight service station when it is in operation.
The now uncommon situation of an On-Airport Flight Service Station as noted in AIM 4-1-3 would seem to be in force here, but in Ketchikan, it appears that under the SFAR, seen in Ketchican FSS Information, participation is mandatory.
With mandatory participation, what separates Ketchikan from an airport which would normally be served with a control tower?  Does it not quite qualify based on traffic?

Because they have an AFIS (aerodrome flight information service), which is similar to a tower, but only provides information, not ATC. This is very common (at least outside the US) at airports which are too small to need ATC, but still too busy to just be uncontrolled. Radio communication is still mandatory, but flights do not receive a clearance and no separation is provided. The AFIS operators are physically located in a tower, by the way, just like ATC would be.

The Aerodrome Flight Information Service (AFIS) and the Flight Service Station (FSS) are essentially a Flight Information Service provided at an aerodrome. It is a higher level of service than A/G radio; however it remains fundamentally a source of information rather than control. Generally, they will pass more comprehensive information on traffic than an A/G station would. In the UK, AFIS do issue mandatory instructions to aircraft and vehicles on the ground, up until aircraft pass a runway holding point. In Canada, FSS will accept flight plans and can relay IFR clearances to aircraft on the ground or in flight. The normal callsign for a FFS is "radio", for example "Resolute Radio" whilst for an AFIS it is “Information”, for example “Duxford Information”. In both environments, it remains a pilot responsibility to be satisfied that every action is safe and to announce their position and intentions while operating at the aerodrome.

From SKYbrary

Maybe worth noting that this type of service is extremely common outside the US, but quite rare within the US (particularly the lower 48).

A tower can only control what it can see, visually or with radar. The traffic in that area is at low altitude and landing or taking off many places that are not the central airport, much is VFR and not inside the space of a class D tower.


text:
Are there any privately owned large commercial airports?
Do any large, commercial airports exist that aren’t owned and run by the city government they exist in/near?
To provide context, I’m talking major airports that handle commercial airline volume. Do any (hub or regional) airports exist that are fully owned and operated by private companies vs government authorities? Or is this a total fallacy? I could understand companies big enough to have their own corporate fleet/airstrip, but I’m talking a full-scale airport with airlines the public can use.

Comments are not for extended discussion; this conversation has been moved to chat.

Building an airport is a very expensive endeavor and usually involves government subsidies or is completely done by the local government. In recent decades, most government owned airports in Europe have however been transformed into companies that both own and operate the airport, usually with the government as initial owner of this company. Since this privatization, some governments have sold their shares of the company resulting in completely privately owned airports.
Going through the 20 busiest airports in Europe from Wikipedia's list, the following airports are completely or by majority privately owned:

London Heathrow Airport owned by Heathrow Airport Holdings
London Gatwick Airport with a majority ownership by GIP
Rome Leonardo da Vinci-Fiumicino owned by Aeroporti di Roma (3.8% government owned)
Zürich Airport owned by Flughafen Zürich AG (38.38% government owned)
Copenhagen Airport owned by Københavns Lufthavne (39.2% government owned)
Lisbon Airport owned by Vinci SA, although operated by the government

The following airports are run by a private company, but they are majority government owned:

Paris Charles de Gaulle Airport and Orly Airport owned by Groupe ADP (50.6% state owned)
Amsterdam Airport Schiphol owned by Royal Schiphol Group (92% government owned)
Frankfurt Airport owned by Fraport (51.47% government owned)
Madrid-Barajas, Barcelona-El Prat and Palma de Majorca owned by Aena S.A. (51% owned by ENAIRE, which is a public corporate entity)
Munich Airport owned by Flughafen München GmbH (100% government owned)
Oslo Gardermoen owned by Avinor (100% state owned)
Manchester Airport owned by Manchester Airport Holdings (64.5% government owned)

The following airports are directly owned by the local governments:

Istanbul Atatürk Airport
Dublin Airport

I am not sure who exactly owns Sheremetyevo and Domodedovo in Moscow.

Sheremetyevo is 68% (66%?) owned by TPS Avia. 30.43% is controlled by the state through the Federal Property Management Agency. I’m not sure where the missing few % have gone.

Funnily enough, even the Russian government had troubles figuring out who owns DME, as it appears to be privately-owned through a chain of off-shore companies.

@Tim Yes, but since I couldn't find details about TPS Avia I left it at that. As aland's comment shows, It's complicated :)

Are there any large airports that weren't built by govs?

@DavidGrinberg See Neolan's answer.

You can add London Southend Airport (Don't get me started!) which is owned by Stobart Group

London Heathrow (LHR) is owned by a private company, Heathrow Airport Holdings. I'm sure there are many other examples around the world.

The major Australian airports were all privatized from the late 1990's. Smaller regional airports are still usually council-owned and operated.

The Australian airport "privatisations" should stand as a warning to any jurisdictions hoping to solve short-term government financial problems by selling public assets, usually to favoured insiders. Our airports are now famous for monopolistic price-gouging on an extraordinary scale, while delivering substandard services to both airlines & the travelling public. The owners of YSSY have been quietly sabotaging the building of a non-curfew 2nd airport for nearly 15 years (their 1st CEO was the Prime Minister's ex Chief of Staff) , because they don't want to lose their monopoly.

@Mackk Yeah... I'm normally all for having most industries owned and run privately, but things that inherently lend themselves to monopoly - such as roads, utilities... and airports - are really a different beast. Government is almost always less efficient than private industry when free competition is allowed and feasible, but free competition on major public infrastructure projects like airports and most roads just really isn't feasible. In cases like these where a monopoly is required, having it run by people who must answer to voters is usually the least bad alternative to free market.

@reirab I suppose the way Heathrow works is there are around 6 competing airports around the region, and one especially close. Heathrow still has a number of issues - a lack of connections to the U.K. being one of them - but it’s not the Australia situation by a long stretch.

@Tim Also note that Heathrow is a hub and competes with other European hubs, like Paris, Amsterdam and Frankfurt. If Heathrow comes with incompetitive service and pricing, British Airways goes down together with it, and Air France, KLM and Lufthansa will take over the lion share of that hub traffic.

@Alexander that’s true, although the fact that Schiphol has more flights to the UK than Heathrow is annoying.

@Mackk : "to solve short-term government financial problems by selling public assets, usually to favoured insiders" - this is all too common. An egregious case I've heard about involves a government office, which was in a high value building in a central part of the city. To raise money, the building was sold. While debating on the building to relocate the office, they decided they would "temporarily" rent the building they were in, from their new owners, for a month or two. Practical, right? But after ten years they are still there, having payed back the selling costs 5 times over, as rent.

...and who owns them? Can you list the top-n largest, and their owners?

@tim Note that BAA owned all the sizeable london airports --- Heathrow, Gatwick, Stansted --- and was forced in 2009 to sell them off  (sold G but appealed on S, lost 2011). e.g. https://www.bbc.co.uk/news/business-14196639 . After that there was no point in being called BAA anymore and renamed to Heathrow.

Speaking of Sheremetyevo to continue @Bianfable answer, as of 23 April 2019:

66% are held by Sheremetyevo Holding which it turn is owned by an offshore cypriot TPS Avia Holding. TPS Avia Holding in turn: 65,22% are held by trust of Ponomaryenko abd Skorobogatko families and 34,78% Arkadiy Rotenberg. While that seems to be private owners, they are tightly connected to the Russian government and especially Vladimir Putin.
30,43% are off to Federal Property Management Agency
Minorities are Aeroflot Airlines and VEB Capital (bank). Both of those companies are controlled and owned by the Russian government

So on paper Sheremetyevo is a privately owned company, but in fact it is controlled by Russian gov't.
Source in russian: vedomosti.ru

Don't forget Punta Cana Airport, Dominican Republic. It was one of the first privately owned airports in the world.

@bogl Why would it not answer the question? Punta Cana would certainly fall under large commercial airport with connections to 3 continents and according to Wikipedia it is owned by "Punta Cana Resort and Club/Grupo Punta Cana".

This absolutely answers the question: yes -- Punta Cana.  A link to provide substantiation would be nice, but it's an absolutely clear & direct answer.

@bogl then edit it to remove the “don’t forget” -then it’s no-longer addressed at other answers.

So is that why the airport seems so popular, because it was built by a group that owns nearby resorts?

"Don't forget (an airport I've never heard of)"?  Links to three continents is a bit trite, practically any carribean airport links to two (N- and S- America) so just add package flights for EU tourists and you're there... with the same number of passengers as any mediterranean island airport that you'd never list as a major airport.

In New Zealand both Auckland and Wellington airports are privately owned.
In both cases the local government have a non-majority shareholding which provides a fig leaf of representation but in reality nothing other than sincecures for local politicians .
They are de-facto monopolies for the region they serve and manifest the types of behaviour you would expect from that situation. In theory their profits are limited by a formula which is overseen by national government but this just has the effect that they squeeze every last cent out of activities which fall outside of the formula but which they still have a monopoly over.

In the United States, there is only one privately owned and operated airport with scheduled commercial service:  Branson, MO, which only has seasonal service to three other destinations.  The vast majority of airports in the United States are at least publicly owned, and many (the majority, I believe) are publicly operated as well.
The fact that airports in the United States have largely not been privatized while many airports in Europe have been privatized is perhaps a bit surprising, given both the USA's and Europe's relative views on private vs. public enterprise.  The Government Accounting Office actually produced a report on why airport privatization hadn't taken off (so to speak.)


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What is the relationship between aspect ratio and lift-dependent drag?
It has been a long held view that aspect ratio of the wing has a significant role to play in the lift-dependent drag. However, mixed opinions have been expressed on the importance of aspect ratio in reducing lift-dependent drag.
I've read a lot of papers and articles and haven't found an absolute answer! So how does Aspect Ratio really play out with lift-dependent drag?

Welcome to aviation.SE! Does this question help?

Aspect ratio is overrated.
What counts is span loading, i.e. the weight which needs to be supported per unit of span. For more, please read this answer.
Generally, more important than the aspect ratio is the induced drag of a wing which carries a given mass at its center. Now the mass of the wing itself also needs to become small, counterintuitively leading to designs with higher span and a bell-shaped lift distribution.


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Since when are ground-based starting devices no longer in common use? Are there any jetliners flying that would still require them?
Nothing is written in Wikipedia on how the engines of the De Havilland Comet or other similar airliners were started. However, it seems that aircraft of this generation could not start the engines stand-alone without the ground support, and that the Boeing 727 in 1963 was the first jetliner capable of doing this.
In one of the answers Terry describes technicians suffering desperately while attempting to start 747 with the two old ground-based devices. From this it looks like while "theoretically possible", this was a very uncommon and unexpected task for them.
Since when are ground based starting devices no longer in common use? Are there still any jetliners in service that would require them?

What was uncommon about the experience in Harare, Zimbabwe that you reference in your question was the arrival of a 747 with an inoperative APU. If I had to bet money, I would bet that they had never had that happen, or if it had, the captain had enough sense to leave an engine running. Harare's ground crew experience was having to start much smaller engines requiring far less pneumatic power from the ground carts. Thus pneumatic carts (huffers) that had had their output degrade over time still had sufficient output to start smaller engines.

Since when are ground based starting devices no longer in common use? Are there still any jetliners in service that would require them?

As of my retirement in 1999, ground based starting devices were in common but infrequent use. I would suspect that's still true, although hopefully less frequent. Any jetliner engine of size will require a ground pneumatic source to start if the airplane itself cannot supply that with its APU or an already running engine.
Back to my experience in Harare, the first mistake made was that of our dispatch department sending an aircraft there with an inop APU.


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Why don't all GA aircraft have yaw dampers?
Why don't they have yaw dampers if they are always "suffering" from adverse yaw?

You mean aside from the extra cost, complexity, and maintenance required? Yaw damper systems are pretty complicated, a touch of rudder or rudder trim is pretty cheap and easy for a GA pilot to deal with. Small aircraft don't have much of a dutch roll tendency, and passenger ride isn't a concern, so the need for such a complicated and expensive system isn't there.

I think the fact that most GA aircraft are not swept wing is another factor.

A yaw damper is mostly aimed to prevent Dutch roll on long fuselage aircraft.

Because the pilot's leg muscles need exercising.

A yaw damper means added cost, weight, complexity, and one more system that has to be maintained. That's the downside. The upside, for most GA aircraft is that they relieve the pilot of having to make a "coordinated" turn. In other words, having to apply appropriate rudder input along with aileron input. Is that convenience worth the downside when the pilot has internalized doing that without having to think about it? That's a matter of opinion. For myself, a yaw damper in any of the, say, single engine Cessnas would be a waste.
There are some GA aircraft that could benefit from a yaw damper in other than banking into a turn or leveling from a turn. For example, in level flight the old V-tail Bonanza would yaw around the vertical axis in turbulence to an objectionable degree in my opinion. Very uncomfortable, especially to passengers in the back seat. It would have been great to have had a yaw damper.

Its worth noting that some GA aircraft do have a pseudo yaw-damper: An aileron/rudder interconnect that applies some rudder when the ailerons are deflected (usually a spring or bungee). I believe some models of the Beechcraft Bonanza have such an interconnect, though I'm not sure if it's on the V-tails.

@voretaq7 The Erco Aircoupe was famous for its linked rudder and aileron controls.  The linked controls were sold as a safety feature to prevent spins.

@WayneConrad The Ercoupe is an extreme example, but an interesting one: As originally built it had no rudder pedals, and the interconnect between the rudder and the ailerons produced inherently coordinated turns. That plus its limited elevator travel makes it one of the few aircraft certificated as "characteristically incapable of spinning" (because you can't hold it in a stall long enough to develop into a spin).

And some like the SR-22 have a full on yaw damper...

@voretaq7: Hope no Ercoupe pilot ever had to perform a crosswind landing...

@Vikki I think I read somewhere that the main gear on the Ercoupe could caster allowing crabbed landings in a crosswind.

Adverse yaw is the yawing moment that occurs due to aileron deflection.

Yawing moment from aileron deflection: upon a left turn the right aileron deflects downwards and the left one upwards. The downwards deflecting aileron creates more drag friction, pulling the nose of the aeroplane to the right.
Yawing moment from roll velocity. The wing rolling up has a different Angle of Attack than the wing rolling down: the downwards going wing has the lift vector tilted forwards (like in a glider) and vice versa, which creates a yawing moment pulling the aeroplane nose to the right.

So adverse yaw pulls the nose into the opposite direction of where we want to turn, and the effect scales with wing span. One way to counteract this is to couple the rudder to the aileron, and give automatic nose left rudder deflection upon left aileron roll. Some of the Learjets and I'm sure some other aeroplanes have this, a way to automatically fly coordinated turns.
Yaw dampers on smaller planes do not coordinate turns but damp out Dutch rolling: the rudder deflects to counteract yaw velocity. Only airliners have yaw dampers that also coordinate turns.
The thing is that GA aircraft are mostly manually flown, control surfaces are manually deflected. For a yaw damper you need an actuator, so will need to install a powered rudder system. A major cost driver.

"A yaw damper does not coordinate turns but damps out Dutch rolling": That may have been the original intention and the source of the name yaw damper, but every yaw damper system on a modern airliner is performing turn coordination.

@Bianfable noted and included. OP asks about GA aircraft though.

For what I know, the situation is rather the opposite. Larger airplanes (airliners) are typically underdampened in yaw and prone to dutch roll, esp. at higher speeds. They also have higher demands due to passenger comfort issues. So they require (or 'desire') a yaw damper per se. Turn coordination is a bonus. GA airplanes are typically well dampened naturally (except some, partricularly with V-tail) and have much smaller speed range, and for them turn coordination is the main driver.

Yaw dampers prevent dutch roll on swept-wing aircraft.
A single-engine propeller GA aircraft with a straight wing has no need for a yaw damper, since it has good directional stability and doesn't have a tendency to develop dutch roll like a swept-wing aircraft does.
Dutch roll is when the tail of the airplane “wags”/moves left and right as the wings roll.


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How is the emergency locator transmitter of the A320 fixed, activated and carried away?
I've some closely related questions about the emergency locator transmitter, ELT, the "distress beacon", on the Airbus A320. From this question about antennas on the A320:

The ELT antenna seems to be located aft of the cabin.
However the ELT might need to be activated manually in case the acceleration sensor doesn't work. More importantly the ELT must be taken away if the crew/passengers decide to move away from the aircraft wreckage, e.g. after a ditching.
It could be interesting to know some details before being the only survivor of a A320 crash in the middle of the Amazon rainforest:

Were is the ELT located and how is it fixed?
How is it manually powered?
How is it connected to the fuselage antenna and to the overhead panel?
Is it possible to remove it from the aircraft and take it away?

"More importantly the ELT must be taken away if the crew/passengers decide to move away from the aircraft... " That isn't the purpose of an ELT, it isn't meant to be removed and carried with you, that is what a PLB is for. I don't know of any ELT that is designed that way.

That is more like a PLB, the ELT built into the aircraft (that uses the external antennas/manual cockpit activation) isn't so easily removed without tools, tools you probably wouldn't have in a crash. From a survival perspective you really don't want to wander too far from the crash site if possible.

It's a self-contained unit with its own battery, GPS transmitter and antenna, with a port that can be plugged into the antenna connector of the aircraft. Just disconnect, stick out the whip antenna, and swim downstream the Amazon :)
Image source
Here is Australia boats are required to bring a similar device on board called an EPIRB if going off-shore for more than a couple of miles, also a really good idea if driving from Darwin to Adelaide with a detour through the desert.


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Can a low flying jet fighter lift water?
Can a low flying but super fast jet spray water from the surface of the ocean?
I've seen this visual effect done many times in movies. It's often done to highlight supersonic speed just a few feet above the water.
Do the laws of physics even come close to this being plausible?
Bonus: Can you provide photograph evidence of this happening?

Comments are not for extended discussion; this conversation has been moved to chat.

And this video gives quite the explanation on how this happens...
The way it works:
condensation effects:

the air speed increases as the air flows around the aircraft
this can mean, that the air may travel faster than the speed of sound
air pressure and temperature drop, when the airspeed increases
this leads to condensation, because the cold air cannot hold the amount of water anymore
the cloud the aircraft is seemingly dragging along are condensation effects
the cloud is a local effect, it is not dragged along

shock waves:

they are created, when the air cannot get out of the way of the aircraft fast enough
this way, they form a pressure wave
that is the sound the aircraft makes
generally two waves, one bow and one tail

the rocket at the end:

the air is cold
there are tiny ice crystals in the clouds
they make the two pressure waves of the rocket visible

The best is the visual shockwave at the launch of an atlas rocket at the end of the video!!!

Please "bring the content here" by providing an explanation in the answer.

This answer, while detailed, does not actually answer the question. Do real airplanes create such fumes as in the rendered image from the OP, or do they not?

To quote my comment from the other answer: "The vapor cone is generated from the pressure wave of the aircraft, same as the trail on the water."

I've wondered about this, too.  From what I've discovered looking around online (and I am NOT a pilot or physicist) is that it's something of a myth -- at least in terms of the dramatic waterspout-effect like in the painting above and countless movies.
A fast, low-flying jet may create some shockwaves (not to be confused with the "vapor cone") that would disturb the surface of water, or some updraft that sucks up spray, but nowhere near the massive amount of water depicted.
The idea may have started because of images like below, where a Blue Angle flies low over water then, as it pulls up, the jet engines directly angled toward the water do indeed blast a huge spray -- but that's very different than sucking up water in level flight.
So, the answer is: yes, a low-flying jet can disturb the surface of water, but not quite in the way it's depicted in the movies.
https://www.military.com/video/aircraft/jet-fighters/blue-angels-low-pass-over-water/1090049790001


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How does the idle mixture adjustment in a Stromberg NA-R9B float-type carburetor work?
At what temperature should idle mixture adjustments be made in Stormberg NA-R9B float-type carburetor?

The temp is nominally 68F, but the acceptable range is 40 to 80 ambient, by practice.
I can't say about that specific carb, but generally in the Stromberg and the SUs, there is a needle, which can be raised and lowered, which changed the metering.  By changing the taper of the needle, you can also change the mixture profile through the venturi flow rates.
We used to use a manometer to get the vac reading, and then look at EGT to make a decision on mixture.
The needle height will impact the overall mixture, and the shape of the needle will impact the mixture through the flow range.
What aircarft is this on?  I know some of the J-3s had Stromberg carbs, but this specific carb model is not ringing any bells on aircraft.

Are you sure you aren't looking at a NA-R9B???

My bad, it is NA-R9B. "B" looked like 8 on my paper. Sorry about that.

Kevin, there are sources for the maintenance manual for this carb.  If it is on a plane that will be flown, someone should have the manual. Try this place:  http://www.timetravelair.com/techlibrary/EO%2015-10BA-2.pdf

ALSO, looking at the above referenced manual, this is not a variable venturi carb, which I initially thought it was.  So disregard my talk about the needles for mixture adjustment.  At least we now know the correct carb, and have a manual for it.  Sorry it took a little to get there.

Is okay. Appreciate for the help. Really need it for the verbal.

Can I adjust the idle mixture adjustment based on the cylinder head temperature?

No. That doesn't give an adequate reading of the combustion process, and for example, may not differentiate between excessively rich and excessively lean mixtures.  Check the manual I linked to you, that should cover the manufacturer's approved setup process for that carb.


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How does the idle speed adjustment in a Stromberg NA-R9B float-type carburetor work?
What is the purpose of the idle speed adjustment in a Stromberg NA-R9B float-type carburetor? In what situation do we adjust the idle speed?

Welcome to aviation.SE!

Sorry, my first time using this website.

The idle speed adjustment in any engine is to control the minimum RPM that an engine reaches, in other words how quickly it will run with the throttle all the way off. You want the idle speed to be high enough that the engine will not stop running when the throttle is pulled back, but not so high that it wastes fuel. On a carbureted engine the idle speed is typically adjusted using a set screw which allows a mechanic to quickly adjust the idle speed. This would be done when the airplane is serviced as part of its regular maintenance, or out of the service cycle if there's a reported problem.
The idle speed adjustment is unrelated to the mixture control and is not adjustable in flight.


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Wouldn't a Snow Globe prevent pilots from getting disoriented?
Several times in aviation history a passenger aircraft has crashed just because the pilots were disoriented. They simply just did not understand what was the position of the aircraft that moment.
Wouldn't a Snow Globe prevent this?
There are 'earth' and 'snow'. Their position inside the glass bubble would give better clue of the position of the aircraft than current mechanisms.
One could also design a specified version for aircrafts. Maybe just bubble of glass filled with pebbles.
I think it would work brilliant as artificial horizon.
Has there ever been an experiment with 'snow bubble' as artificial horizon?

are you suggesting to install an artifical horizon/attitude indicator?

You'd be better asking in Physics.SE why it won't work.  See here and here amongst others.

This is a question that is very commonly asked by well-meaning newcomers to aviation. I think the downvotes are a little harsh.

@Ben Agreed.  This community can be stupid sometimes.  How many newcomers have been scared away?  +1 just to do my bit to rebalance the Universe.

@Ben I can understand the downvotes as this reads a bit as a joke. The OP claims some level of knowledge of "current mechanisms" which he or she had determined would be inferior to a snow globe in the cockpit.

Somewhat related (but not a duplicate by my estimation): Would mechanical indicators have helped in the Air France 447 incident?

This doesn't work. A plane can be in arbitrary orientations while the the forces felt by everybody in the plane are directed straight at the floor. A famous example is Bob Hoover pouring tea while rolling a plane. If Bob Hoover's tea can't tell that it's being poured upwards, nor can your snowglobe.  Indeed, this is precisely why pilots become disoriented.